This series and parallel sensor circuit was found in Popular Electronics from the 1990s. Any SCR can be used and the supply voltage depends on the load.
This 1969 Philips documentation circuit makes it possible to control an engine through the sound...
We found this circuit in a 90's Radio Electronics. The circuit can still be easily assembled as all...
This circuit was found in the July 1992 Radio Electronics magazine. Sensitivity adjustment is made...